This invention relates to a method of compensating for a nonlinear distortion of a power amplifier of a transmission section of a wireless communication apparatus, and a wireless communication apparatus which compensates for a nonlinear distortion of a power amplifier of a transmission section.
In a digital wireless communication apparatus at present, the demand for linearity of a power amplifier for transmission becomes severe as increase of the speed and capacity of communication proceeds. However, if the demand for linearity of a power amplifier becomes severe, then the power efficiency of the power amplifier decreases, and this prevents, for example, elongation of the time within which a terminal telephone set of a digital portable telephone system can enjoy a continuous service. Therefore, it has been proposed to compensate for a nonlinear distortion of a power amplifier to augment the power efficiency of the power amplifier.
FIG. 14 illustrates a conventional distortion compensation method disclosed in Japanese Patent No. 2,689,011, xe2x80x9cLinear Transmission Apparatusxe2x80x9d. Referring to FIG. 14, an input signal Si of a modulated wave from an input terminal 101 is inputted to a power amplifier 91 through a directional coupler 92 and a power control circuit 93. An output signal of the power amplifier 91 is led out as an output signal So to an output terminal 102 through another directional coupler 94.
Then, the input signal Si is branched by the directional coupler 92 and supplied to an envelope detection circuit 95, by which an envelope of the input signal Si is detected. The envelope detection signal of the envelope detection circuit 95 is supplied to a dc voltage conversion circuit 96, by which a dc voltage from a power supply terminal 103 is converted in accordance with the envelope of the input signal Si. A dc voltage after the conversion of the dc voltage conversion circuit 96 is supplied as a power supply voltage to the power amplifier 91.
Further, the output signal So is branched by the directional coupler 94 and supplied to another envelope detection circuit 97, by which an envelope of the output signal So is detected. The envelope detection signal of the envelope detection circuit 97 is supplied together with the envelope detection signal from the envelope detection circuit 95 to a difference signal production circuit 98. The difference signal production circuit 98 produces a signal of a difference between the envelope of the input signal Si and the envelope of the output signal So. The difference signal of the difference signal production circuit 98 is supplied through a dc amplifier 99 to the power control circuit 93. The power control circuit 93 controls the input power of the power amplifier 91 in accordance with the difference between the envelope of the input signal Si and the envelope of the output signal So.
According to this method, the power supply voltage of the power amplifier 91 is controlled in accordance with the envelope of the input signal Si to decrease a nonlinear distortion of the input signal Si, and the input power of the power amplifier 91 is controlled in accordance with the difference between the envelope of the input signal Si and the envelope of the output signal So to absorb a gain variation of the power amplifier 91 which is caused by a temperature variation or the like.
With the conventional distortion compensation method described above, however, since the power supply voltage of the power amplifier 91 is modulated by the dc voltage conversion circuit 96 to amplitude modulate the signal to be amplified by the power amplifier 91, there is a drawback that a distortion by the modulation is produced. Besides, since the power supply voltage of the power amplifier 91 is modulated in accordance with the envelope of the input signal Si, there is another drawback that a nonlinear distortion which cannot be detected from the envelope of the input signal Si and does not rely upon the power supply voltage of the power amplifier 91, for example, a distortion which originates from a nonlinear parameter such as a parasitic capacitance Cgs between the gate and the source of a GaAs FET which forms the power amplifier 91, cannot be reduced.
It is an object of the present invention to provide a distortion compensation method and a wireless communication apparatus which can reduce a nonlinear distortion of a power amplifier without producing a distortion by modulation and reduce a nonlinear distortion, which cannot be detected from an envelope of an input signal and does not rely upon the power supply voltage of the power amplifier, with certainty.
In order to attain the object described above, according to the present invention, there is provided a distortion compensation method for compensating for a nonlinear distortion of a power amplifier, comprising a detection step of detecting envelopes of an input signal and an output signal of the power amplifier individually, and an addition step of adding a signal corresponding to a finite difference between the detection signals of the envelopes obtained by the detection step to the input signal of the power amplifier.
In the distortion compensation method, not a signal obtained by modulating a power supply voltage of the power amplifier and amplifying the modulated power supply voltage by the power amplifier is amplitude modulated, but a signal for compensation is added to an input signal of the power amplifier. Consequently, a nonlinear distortion of the power amplifier can be reduced without production of a distortion by modulation. Besides, since a signal corresponding to a finite difference between an envelope of the input signal and an envelope of an output signal of the power amplifier is added as the signal for compensation, also a nonlinear distortion which cannot be detected from the envelope of the input signal and does not rely upon the power supply voltage of the power amplifier can be reduced with certainty.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.